A magnetic random access memory (MRAM) can perform high-speed and substantially infinite (1015 times or more) rewriting because data is stored on the basis of a magnetization direction of a magnetic body, and the MRAM is already used in the fields of industrial automation, airplanes, and the like. Additionally, the MRAM is expected to have further development in code storage and a working memory in future because of its high-speed operation and high reliability, but in reality, there is a problem in reducing electric power consumption and increasing capacity. This is a substantive problem originated from a recording principle of the MRAM, more specifically, a method in which magnetization is reversed by a current magnetic field generated from wiring.
As a method to solve the above-described problem, a recording method without using the current magnetic field, more specifically, a magnetization reversal method is studied, and particularly, attention is paid to a spin transfer torque based magnetic random access memory (STT-MRAM) that applies magnetization reversal by spin injection (refer to, for example, Japanese Patent Application Laid-Open No. 2014-072393).
The magnetization reversal by spin injection is a phenomenon in which electrons that have been spin-polarized by passing through a magnetic body are injected into the other magnetic body, thereby causing magnetization reversal in the other magnetic body. Compared to the MRAM in which magnetization reversal is performed on the basis of an external magnetic field by utilizing magnetization reversal by spin injection, the spin transfer torque based magnetic random access memory has advantages in which: write current is not increased even when a device size is miniaturized; scaling can be performed because a write current value is reduced in proportion to the volume of the device; a cell area can be reduced, and furthermore, there is an additional advantage in which a device structure and a cell structure are simplified because a word line used to generate recording current magnetic field that is required in the MRAM is unnecessary.
FIG. 4 illustrates an equivalent circuit diagram of a nonvolatile memory cell including: a spin transfer torque based magnetic random access memory that is two-terminal element; and a selection transistor that is a three-terminal element including a gate electrode and source/drain regions. The spin transfer torque based magnetic random access memory includes, for example, a magnetic tunnel junction element (MTJ element) and has at least two magnetic layers (specifically, a storage layer and a magnetization fixed layer). In the magnetization fixed layer, a magnetization direction is fixed. On the other hand, in the storage layer (free layer), the magnetization direction is varied, and information “1” or “0” is stored depending on the magnetization direction. The spin transfer torque based magnetic random access memory has one end connected to one of the source/drain regions (referred to as “drain region” for convenience sake) of the selection transistor TR and has the other end connected to a bit line BL. Additionally, the selection transistor TR has the other one of the source/drain regions (referred to as “source region” for convenience sake) connected to a sense line 65. Furthermore, when current is made to flow from the bit line BL to the sense line 65 or by current is made to flow from the sense line 65 to the bit line BL, magnetization direction of the storage layer is reversed by spin injection in accordance with a flow direction of the current.
In the spin transfer torque based magnetic random access memory utilizing such magnetization reversal by spin injection, voltage and current applied to the spin transfer torque based magnetic random access memory at the time of writing information are determined by drive capacity of the selection transistor. Meanwhile, the drive current in the selection transistor has an asymmetry property in which, for example, a value of flowing current is different between a case where the current flows from the drain region to the source region and a case where the current flows from the source region to the drain region.
FIG. 14A illustrates an equivalent circuit diagram in “writing-1” in which current flows from a sense line to a bit line via the selection transistor and the spin transfer torque based magnetic random access memory, and a relation between voltage applied to the spin transfer torque based magnetic random access memory and the current flowing in the spin transfer torque based magnetic random access memory and the selection transistor. Additionally, FIG. 14B illustrates an equivalent circuit diagram in “writing-2” in which current flows from the bit line to the sense line via the spin transfer torque based magnetic random access memory and the selection transistor, and a relation between the voltage applied to the spin transfer torque based magnetic random access memory and the current flowing in the spin transfer torque based magnetic random access memory and the selection transistor. In each of FIGS. 14A and 14B, a vertical axis represents the current (unit: microampere) flowing in the spin transfer torque based magnetic random access memory and the selection transistor, and a horizontal axis represents the voltage applied to the spin transfer torque based magnetic random access memory (unit: volt). In FIGS. 14A and 14B, assume that: the spin transfer torque based magnetic random access memory is represented as “MTJ”; and the selection transistor includes NMOS. In the example illustrated in FIG. 14A, Vdd (for example, 1.0 volt, and the similar voltage is applied in the following description) is applied to a sense line (source region), and a bit line is grounded. On the other hand, in the example illustrated in FIG. 14B, Vdd is applied to the bit line, and the sense line (source region) is grounded. Additionally, in both cases of writing, the selection transistor is made to a conductive state by applying the power supply voltage Vdd to a gate electrode of the selection transistor, and current is made to flow in the spin transfer torque based magnetic random access memory via the selection transistor. At this point, a current direction is changed depending on whether the power supply voltage Vdd is applied to the sense line or applied to the bit line, and desired information can be written in the spin transfer torque based magnetic random access memory.
Here, a gate potential is fixed at Vdd. Additionally, in the case of “writing-1”, a potential of the drain region has a value between Vdd and VGND, specifically, ΔV because of voltage drop (ΔV) in the spin transfer torque based magnetic random access memory. Therefore, a potential difference ΔV1 between the gate electrode and the drain region becomes (Vdd−ΔV). On the other hand, in the case of “writing-2”, a potential of the source region is fixed at VGND, and a potential difference ΔV2 between the gate electrode and the source region becomes Vdd.